In the completion of wells drilled into the earth, a string of casing is normally run into the well and a cement slurry is flowed into the annulus between the casing string and the wall of the well. The cement slurry is allowed to set and form a cement sheath which bonds the string of casing to the wall of the well. Perforations are provided through the casing and cement sheath adjacent the subsurface formation.
Fluids, such as oil or gas, are produced through these perforations into the well. These produced fluids may carry entrained therein sand, particularly when the subsurface formation is an unconsolidated formation. Produced sand is undesirable for many reasons. It is abrasive to components found within the well, such as tubing, pumps, and valves, and must be removed from the produced fluids at the surface. Further, the produced sand may partially or completely clog the well, substantially inhibiting production, thereby making necessary an expensive workover. In addition, the sand flowing from the subsurface formation may leave therein a cavity which may result in caving of the formation and collapse of the casing.
In order to limit sand production, various techniques have been employed for preventing formation sands from entering the production stream. One such technique, commonly termed "gravel packing", involves the forming of a gravel pack in the well adjacent the entire portion of the formation exposed to the well to form a gravel filter. In a cased perforated well, the gravel may be placed inside the casing adjacent the perforations to form an inside-the-casing gravel pack or may be placed outside the casing and adjacent the formation or may be placed both inside and outside the casing. Various such conventional gravel packing techniques are described in U.S. Pat. Nos. 3,434,540; 3,708,013; 3,756,318; and 3,983,941. Such conventional gravel packing techniques have generally been successful in controlling the flow of sand from the formation into the well.
In U.S. Pat. No. 4,378,845, there is disclosed a special hydraulic fracturing technique which incorporates the gravel packing sand into the fracturing fluid. Normal hydraulic fracturing techniques include injecting a fracturing fluid ("frac fluid") under pressure into the surrounding formation, permitting the well to remain shut in long enough to allow decomposition or "break-back" of the cross-linked gel of the fracturing fluid, and removing the fracturing fluid to thereby stimulate production from the well. Such a fracturing method is effective at placing well sorted sand in vertically oriented fractures. The preferred sand for use in the fracturing fluid is the same sand which would have been selected, as described above, for constructing a gravel pack in the subject pay zone in accordance with prior art techniques. Normally, 20-40 mesh sand will be used; however, depending upon the nature of the particular formation to be subjected to the present treatment, 40-60 or 10-20 mesh sand may be used in the fracturing fluid.
The fracturing sand will be deposited around the outer surface of the borehole casing so that it covers and overlaps each borehole casing perforation. More particularly, at the fracture-borehole casing interface, the sand fill will cover and exceed the width of the casing perforations,and cover and exceed the vertical height of each perforation set. Care is also exercised to ensure that the fracturing sand deposited as the sand fill within the vertical fracture does not wash out during the flow-back and production steps. After completion of the fracturing treatment, fracture closure due to compressive earth stresses holds the fracturing sand in place.
In most reservoirs, a fracturing treatment employing 40-60 mesh gravel pack sand, as in U.S. Pat. No. 4,378,845, will prevent the migration of formation sands into the wellbore. However, in unconsolidated or loosely consolidated formations, such as a low resistivity oil or gas reservoir, clay particles or fines are also present and are attached to the formation sand grains. These clay particles or fines, sometimes called reservoir sands as distinguished from the larger diameter or coarser formation sands, are generally less than 0.1 millimeter in diameter and can comprise as much as 50% or more of the total reservoir components. Such a significant amount of clay particles or fines, being significantly smaller than the gravel packing sand, can migrate into and plug up the gravel packing sand, thereby inhibiting oil or gas production from the reservoir.
A hydraulic fracturing method employing a special sand control technique was disclosed in U.S. Pat. No. 4,549,608. The fracturing fluid utilized contained an agent for stabilizing clay particles or fines along a fracture face. A proppant comprised of gravel packing sand was injected into the fracture. The sand utilized was not suitable for use in certain acid and high temperature environments encountered in some formations.
Therefore, what is needed is a novel sand control method for use in producing an unconsolidated or loosely consolidated oil or gas reservoir which comprises a hydraulic fracturing method that stabilizes the clay particles or fines along the fracture face and which also creates a very fine refractory gravel pack along the length of such fracture face.